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21 Aug 2020

Basics of the Powertrain NVH: Part 1

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Skill-Lync

 

The powertrain of an automobile is the combination of the main moving parts that include the engine and transmission. This blog will hence deal with understanding the concept of powertrain NVH ( Noise, Vibration, Hardness) and how it affects the design configuration of the power train of the vehicle. 

 

The Approach

Noise is an unpleasant, unwanted, and disturbing sound, which is quite subjective. Some perceive the noise as aggravating, while some might not even notice it if they are used to it. 

 

Vibration is a sensation felt by the driver due to a repeated movement of the component they are in contact with. Vibration is based on the principle of superposition. The steering wheel in a stationary car with running engines is a pretty good example.

 

Harshness is the combination or a result of the effects of noise and vibrations when the vehicle is functioning.

 

NVH can be perceived by the sense of touch, hearing, and vision. It is felt and heard by the vibrations in the steering wheel, and the vision is affected by the rearview mirror's movement. 

 

The Human Ear Proposition

 

 

The graph shows the limitations of the human ear's audibility that are below the 100 Hz range compared to the intensity. On the other hand, even low-intensity sounds can be heard at higher frequencies. 

 

 

The above illustration shows a relative comparison between sound pressure (measured in Pascals) and sound pressure level (measured in Decibels). The sound pressure level is used as a primary tool to determine the intensity of loudness emerging from a source. 

The human ear can withstand 140dB at most; anything more intense can cause permanent damage. The sound of a jet plane taking-off falls in that range. Usually, 60dB is taken as the average sound level before any pain is experienced, which is the degree of sound in a traffic jam. 

 

NVH Characteristics

NVH is a by-product of a broken or worn-out component. It is an unavoidable phenomenon which can be reduced, but not neglected. Every single object with a definite mass or stiffness has a natural frequency associated with it, which will possess its vibration characteristics.

The best example to study powertrain NVH characteristics is the simple spring-mass model.  

 

 

When the weight is attached to the spring without applying any external forces to disturb the equilibrium, it remains stationary. As soon as the weight moves towards the opposite direction, the spring vibrates along a path in the same plane, exhibiting its vibration characteristics. 

The exact concept can be applied to cars, where the mass or weight is the car itself, and the spring is the suspension, as shown above. 

 

NVH Terminologies

  1. Cycle: The repetitive movement of mass with respect to the time domain.
  2. Frequency: The summation of cycles undergone in one second.
  3. Amplitude: The amount of movement of the vibrating system.
  4. Resonance: The summation of the frequency of the vibrating force and the natural frequency of the system, which results in a larger amplitude. This phenomenon is commonly observed in buses where sometimes, the vibration keeps accumulating and increasing as time goes by.
  5. Damping: The natural tendency of the amplitude of the oscillations to decrease as energy drains out from the system, resulting in the resonance being terminated early on.

 

 

If a system is in resonance, the intensity of the vibration keeps growing, which will cause harm to the components. Hence it is primarily essential to damp these vibrations. 

 

Time Domain Vs. Frequency Domain

To better understand this concept, consider the musical instrument - Flute.

 

 

The graph illustrated represents the development of the sound emerging from the flute. The first graph compares the amplitude of the sound with respect to time, while the second one examines the amplitude with respect to frequency

This same concept can be applied to the domain of the automobile. Consider a car's differential. It experiences a tremendous amount of vibrations, which can be represented in the same manner. 

 

 

The frequency of the vibrations can be due to multiple components in the differential, such as the ring gear, the axle shaft, etc. 

Hence, when analyzing the Time Domain, the graph will include a combination of the effects due to the individual components.

The Frequency Domain, on the other hand, will depict the contributions of each component, respectively. 

 

The Powertrain

The powertrain is the combination of all the essential moving parts that make up the powerplant of the vehicle that generates and delivers power. The powertrain consists of the engine, transmission, drive shafts and differentials. These components can differ based on the type of vehicle. (AWD, electric, etc.)

The powertrain has two primary configurations depending on the way the crankshaft sits relative to the rest of the car.

 

  • North-South AWD

This style of arranging the engines parallel to the car's direction can be found on rear-wheel and four-wheel drives.

In this configuration, the engine is placed along the centerline with the gearbox behind it. This placement allows the driveshaft to run from the transmission to the rear differential image l. 

 

 

  • East-West AWD

This style of arranging the engines perpendicular to the car's direction can be found in rear-wheel drives too, but this allows for a more compact design than the other configuration. 

In general, the engine is mounted with a gearbox to one side. You will find it positioned in line with the crankshaft and between the front wheels. This arrangement is known as a transaxle.

 

 

Analyzing the Noise Sources and Excitations

The following are the dominant noise sources in the powertrain:

  • Engine noise originates from both; the combustion process and the mechanical forces associated with the engine dynamics.
  • The rotatory machinery (torque and inertial forces) also generates a significant amount of noise.
  • The shocks due to combustion
  • Mechanical noises due to the vibrations caused by the gears and belts
  • Idle rpm vibrations
  • The ignition and switch off of the engines
  • The unbalanced rotation of the cooling fan

 

 

The noises are sub-categorized into airborne and structure-borne. Airborne noise is a result of external factors like wind, while structure-borne is due to the engine parts mentioned above. 

 

Conclusion

The most critical component in any vehicle is the powertrain, without which the machine is nothing but a soulless chassis. Many factors determine a vehicle's performance, with the configuration of the powertrain being the most crucial. Learn how every vehicle differs based on its performance in the continuation of this blog.

If you want to learn more about how powertrain functions and start your career as a mechanical or automobile engineer, you can click here to check our course about powertrains.  


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Akhil VausdevH


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Skill-Lync

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